In general, a solar cell is manufactured using a silicon wafer or other semiconductor wafer. In order to improve conversion efficiency of the solar cell, it is necessary to reduce the light that reflects on a light receiving surface of the solar cell and the light that passes through the solar cell. For example, in preparing a crystalline solar cell using the silicon wafer, since the silicon wafer has a low transmittance of visible light contributing to photoelectric conversion, in order to improve the conversion efficiency, it is important to reduce reflection loss of the visible light on the silicon wafer surface acting as the light receiving surface, thereby effectively capturing incident light within the solar cell.
Techniques for reducing the reflection loss of the incident light on the silicon wafer surface include a technique of forming an antireflection film on the surface and a technique of forming, in the surface, an uneven structure such as micro-sized pyramidal irregularities called a textured structure. As for the latter technique, the method of forming the textured structure in the surface is suitable for single crystal silicon and represented by a method of etching the single crystal silicon (100) surface with alkaline solution. This method leverages an etching rate of the (111) surface that is lower than that of the (100) surface and the (110) surface. As the latter technique, isotropic etching with acid solution is also known. The etching with an acid solution uses acid containing nitric acid and hydrofluoric acid, thereby making a reaction in which the surface of silicon is oxidized to form SiO2, and then the SiO2 is dissolved with hydrofluoric acid. When a polycrystalline silicon wafer having a surface with non-uniform crystal orientation is etched with an alkaline solution, the textured structure can be formed only on crystal grains having a (100) surface exposed in the wafer surface, and the textured structure cannot be sufficiently formed on the other crystal grains. Accordingly, the uneven structure is formed in the polycrystalline silicon wafer mainly by etching with an acid solution. In this specification, the treatment of etching a surface of a polycrystalline wafer with acid solution in order to reduce the reflection loss is hereinafter referred to as “acid texturing”.
Here, focusing on that a sufficient fill factor cannot be obtained by only etching a surface of a polycrystalline silicon substrate with an acid solution, JP 2005-136081 A (PTL 1) discloses a method of producing a solar cell wafer, including a step of etching the surface of the polycrystalline silicon substrate with an alkaline solution such as NaOH to 7 μm or more before acid texturing. A silicon wafer has cutting damage on its surface immediately after being cut from an ingot. The etching with an alkaline solution in PTL 1 is performed to an extent such that the damage due to the slicing process can be removed. Specifically, after removing the damage due to slicing by alkali treatment, an uneven structure is formed by etching with an acid solution, thereby manufacturing solar cells using this wafer as their substrate.
Here, the methods of cutting a polycrystalline semiconductor block for obtaining a polycrystalline semiconductor wafer can be broadly classified into two types: methods using free abrasive grains (free abrasive cutting) and methods using bonded abrasive grains (bonded abrasive cutting). In the free abrasive cutting, a slurry containing abrasive grains is used as a working fluid, and a wire runs while continuously supplying the slurry to the wire. The polycrystalline semiconductor block is cut by the grinding action of the slurry fed by the running wire to the portion to be cut. However, the free abrasive cutting has some problems such as low slicing speed, since slurry is used as a working fluid. Hence, in recent years, the bonded abrasive cutting for cutting a polycrystalline semiconductor block using a bonded abrasive wire has been attracting attention. In the bonded abrasive cutting, slicing is performed using a wire to which abrasive grains are fixed with a resin or by electroplating (which is called “bonded abrasive wire”). Specifically, using the bonded abrasive wire, the polycrystalline semiconductor block is sliced by the grinding action of the abrasive grains fixed to the surface of the wire. Accordingly, a working fluid (coolant) free of abrasive grains can be used, which can solve the problem of the free abrasive wire, caused by slurry.